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Power Transmission


Fig. 1. Wilhelm Stoll’s front loaders are often used in continuous operation and under extreme conditions.


4 Machine builders, material fabricators, hydraulic and pneumatic equipment manufacturers and robotics users all have to grapple with the issue of controlling the brute force of hydraulic and pneumatic cylinders and actuators. Boris Sedacca reports.


Fluid power adds muscle to robotics and mobile equipment


F 28 www.engineerlive.com


luid power has been used since ancient times but recent developments have seen a demand for safer, quieter pneumatic systems and leak-proof hydraulics. The sometimes brute force


of fluid power needs to be tamed and accurately targeted to applications. Today’s sophisticated feedback control valves


have come a long way from the days of early fluid power systems, which were either on with full flow, or off with no flow. Flow control was achieved by choking off the output with a faucet, lever or similar mechanism to reach the desired flow. Then along came proportional servo valves which controlled flow at the input using the inherent mechanical features of the valve. A key technological breakthrough came later


with the use of electrical control in the form of the solenoid. Since then, the sophistication and complexity of valves has evolved at a dizzying pace leading to the latest proportional integral derivative (PID) electronic feedback devices and computer based systems of valve islands linked by fieldbus networking.


Hydraulic consultant Stephen Barrett says the boundary between mechanical, electrical and control technologies have become so blurred as to be indistinguishable. “Clearly cost comes into it because one of the main requirements is accuracy, and generally speaking, you get the best accuracy with electronic control,” explains Barrett. “Embedded electronics also simplifies the


mechanical complexity of a valve, so it is possible to have closed loop control system that comprises only of the cylinder or actuator, and the valve. You need a command signal, which at its simplest would be a lever, knob or slider, with closed loop control inside the actuator.


Explosive atmospheres


“At the other end of the scale, mechanical feedback systems are simple and reliable, and may be needed in explosive atmospheres (ATEX) for example, where the use of electricity is an issue. Generally, there is more intrinsic safety in a mechanical setup where an actuator is moving until it hits a mechanically operated cam instead of an electrically operated device.


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